Transcription initiation of the Mg2+ transport protein MgtA is regulated by the PhoP-PhoQ two-component system in Salmonella. However, a second regulation event takes place in the cytoplasm during transcription elongation. Recently, 5'-untranslated regions (UTRs) of mRNAs have been identified as cis regulators of expression. These regions, called riboswitches, change conformation upon metabolite binding and prevent transcription elongation through the coding region or translation initiation. The 5'-UTR of the mgtA mRNA shares some structural features of other known riboswitches, however in this case the metabolite appears to be a Mg2+ ion. Preliminary work confirms that the mgtA 5'-UTR responds to Mg2+ concentrations and regulates MgtA expression in Salmonella. We will determine the structures of the RNA conformations in the riboswitch using chemical and enzymatic probing reagents, fluorescence spectroscopy, NMR spectroscopy, and X-ray crystallography. In addition to these methods, hydroxyl radical footprinting will be used to help location the regulatory Mg2+ binding sites. Together with analysis of expression levels in vivo, we will elucidate the roles of various components of the riboswitch to better understand its regulatory mechanism. [unreadable] [unreadable] [unreadable]